It’s extremely hard to do the final edit of your own work. Our brains put in the missing words and correct the spellings while we don’t even notice them. The author can read a paragraph a dozen times and miss an obvious mistake. Whatever you decide to do it’s well worth asking a friend or acquaintance to read through your work and pick out any obvious mistakes.
Editing can be more than this if you wish. A good editor along with finding the spelling and grammar problems can sometimes even help with the storyline including finding where discontinuities exist. The most common problem is probably the writer’s temptation to use three words where just one would do fine and if you want a full edit, the editor will find this for you.
If you need help with editing, a basic edit/proofreading from an experienced editor would cost 300 Thai Baht per 1000 words. (About US$10 per 1000 words.)